2-(1-{6-[(2-[fluorine-18]fluoroethyl)(methyl)amino]-2-naphthyl}-ethylidene)malononitrile (FDDNP) is the first positron emission tomography (PET) molecular imaging probe to visualize Alzheimer's disease (AD) pathology in living humans. The most unique features of FDDNP are that (1) it is the only currently available radiotracer to image neurofibrillary tangles, beside amyloid aggregates, in living humans; and (2) it is also the only radiotracer to visualize AD pathology in the hippocampal region of living humans. In this article, we discuss FDDNP's unique ability to image tau pathology in living humans. Emphasizing tau pathology imaging capability using FDDNP in AD, as well as other tauopathies, is timely and beneficial considering that (1) post mortem histopathological studies using human specimens have consistently demonstrated that neurofibrillary tangles, compared with amyloid plaques, are better correlated with the disease severity and neuronal death; and (2) recently reported clinical trial failures of disease-modifying drugs in development, based on the amyloid-cascade hypothesis, suggest that some of the basic assumptions of AD causality warrant reassessment and redirection.